TOP267 Startup Issue

Posted by: davidsmith52 on 10/19/2011

I'm trying to diagnose a startup problem. The control voltage seems to ramp down much too quickly, reaching 4.8V long before the feedback loop engages. My understanding from the datasheet is that this timing is controlled internally and should take 17mS, but I'm seeing less than 1mS.

The cycle seems to repeat 2-3 times per second.

I have grounded V and X for troubleshooting. I also removed the bias rectifier and tried an external bias supply just to eliminate that variable.

The thing will start into a light load. I can then load it down and it operates and regulates just fine, so the feedback loop operates properly. It just never gets a change to engage.

Maybe I'm missing something about how the softstart works with this part.

Any ideas would be much appreciated.

Attachments:

waveform at control pin during failed startup
schematic of control pin circuit

Files

failed startup_0.gif (18.66 KB)

control pin_0.pdf (22.63 KB)

Comments

I can work around this problem with an external softstart circuit that engages the feedback loop during voltage ramp-up when starting. Works every time.

So, there must be something about the internal softstart that I have not been able to glean from the datasheet. Has anyone else experienced this?

Hi,

I have observed that the value of R151 is 6.8K Ohms in your schematic, where as the value of R151 should be close to 6.8 Ohms. Could you please change that resistor to 6.8 Ohms and see what happens with the start up. Let me know if it doesn't start up.

Regards,
PI-NANO

Yes, you are correct.

I've done some experiments. With 6.8 ohms I don't need the external softstart. However, the transient load response is poorer, in that the voltage excursion is larger with 6.8 ohms than with 6.8K. Makes some sense, as the AC loop gain should be lower with the lower resistance.